Magnetic resonance imaging (MRI) is a well-established medical imaging and diagnostic tool. A great deal of current activity and research relates to interventional and/or intraoperative procedures conducted under MRI guidance (iMRI). In many interventional and intraoperative procedures under MRI guidance, surgical devices such as long needles, guidewires, and catheters are inserted into a subject undergoing MRI analysis. Such procedures enable surgeons to image, locate, and track such devices during a MRI procedure. To enable such tracking in real time during a MRI procedure, interventional devices are typically equipped with a radio frequency (RF) antenna, more particularly an RF coil.
The RF coil is typically located at the distal end of the catheter or other device and receives a RF signal emitted from excited protons of blood or tissue in its vicinity when they return to equilibrium. The RF coils sends an electrical signal directly to the MRI scanner by way of an attached coaxial cable. The coaxial cable is typically a very thin coaxial cable that runs through a lumen in the catheter. The presence of long conductive objects, such as coaxial cables, has been found to lead to heating at the tip of the device. Medical studies indicate that this effect is due to coupling of the RF field from the MRI system, primarily to long cables or transmission lines, i.e., longer than one quarter of the RF wavelength within the body (approximately 80 cm), couples significantly with the RF transmission energy of the body coil of the MRI system. Examples articles that discuss such studies include “Reduction of Resonant RF Heating in Intravascular Catheters Using Coaxial Chokes”, Mark E. Ladd et al., Magnetic Resonance in Medicine 43:61-5-619 (2000); “RF Safety of Wires in Interventional MRI: Using a Safety Index”, Christopher J. YEUNG et al., Magnetic Resonance in Medicine 47:187-193 (2002); “RF Heating Due to Conductive Wires During MRI Depends on the Phase Distribution of the Transmit Field”, Christopher J. YEUNG et al., Magnetic Resonance in Medicine 48:1096-1098 (2002); and “Safety of MRI-Guided Endovascular Guidewire Applications”, Chia-Ying LIU et al. Journal of Magnetic Resonance Imaging 12:75-78 (2000)). These studies indicate that long transmission cables, even without the RF coil, show significant heating, whereas, RF coils without the cable show no heating.
Although decoupling circuits have been used at the proximal end of the catheter to reduce the electric field coupling, such circuits do not work well when the conductor length increases (e.g., exceeds 80 cm). Because of this heating problem, active MRI compatible and visible catheters are currently used only in animal studies. Consequently, there is a need for improved active MRI compatible devices that do not have a severe heating problem.